"Good morning" is said at night

When we start to learn Japanese language, we will first learn the Japanese greetings.
The very first Japanese greeting would probably be おはようございます ohayou gozaimasu "Good morning".
We were taught that ohayou gozaimasu would simply mean good morning and it should be used in the morning.

When I was hired to work in the 居酒屋 izakaya (Japanese pub) as a アルバイト arubaito part-timer, I wanted to give a good impression on the first day at work. So I went to work early while other workers have not arrived yet. At 4pm I arrived at work and said aloud, "こんばんわ konbanwa Good evening" to the 店長 tenchou shop manager and assistant shop manager. But they just うなづいたunaduita nodded quietly in return. At that time, I thought I gave a proper あいさつ aisatsu greeting, thus I felt it was okay. However, after that, when the other workers came, each of them greeted by saying, "Ohayou gozaimasu, ohayou gozaimasu!".

"Huh, it is already past 5pm. Why do they say ohayou gozaimasu?" I was confused.
Out of curiosity, I asked another non-Japanese worker there,"Hey, why did you said ohayou gozaimasu?"
"Hmmmm....every part timer need to say ohayou gozaimasu. Regardless if it is morning, noon, or night, you say `good morning`. I also don`t know, just say it," he explained.
Although I can`t really get it, I just replied,"なるほど naruhodo Oh, I see. From now on I will greet ohayou gozaimasu!".

Arigatou vs Yoroshiku onegaishimasu

In 英語 eigo English, there is no greeting equivalent to "yoroshiku onegaishimasu". The closest translation on the web would be "Please take good care of me". It is mostly used during 自己紹介 jiko shoukai self-introduction. I didn`t know the yoroshiku onegaishimasu for other uses. For example, I didn`t know that yoroshiku onegaishimasu can be used in writing emails too.

In English business emails, for requests, we would end with "Thank you" and "Regards".
I got used to that way of writing, thus when I was writing Japanese emails, I would end by writing "arigatou gozaimasu". I continued doing it for some time until one day, my 先生 sensei teacher found out and told me: "When you have any request, say yoroshiku onegaishimasu".

Different from English emails, in Japanese language, there is a specific word when requesting. You must use yoroshiku onegaishimasu. Suddenly, I felt I was 失礼 shitsurei impolite since I didn`t use the yoroshiku onegaishimasu before.
When you are requesting your counterpart, and while your counterpart haven`t said if he can or cannot do it, saying "arigatou gozaimasu" would means that you have already assumed that your counterpart will definitely able to do it. Although no one had 文句を言う monku wo iu complained me, to a certain extent, I probably had been impolite to some people.